Sewing machines that form stitches on a workpiece cloth with a needle thread and a bobbin thread are generally provided with a thread tension unit, which adjusts needle thread tension provided on a needle thread path of a sewing machine arm. The thread tension unit, being operated by the user, optimizes the tension balance, which is a balance in tension exerted on the needle thread and the bobbin thread of stitches formed on the workpiece cloth to allow smooth and precise sewing operation. Conventionally, adjustment in tension balance at thread tension unit has been performed by evaluating the tension balance through visual observation of test stitches, for example, formed on the workpiece cloth.
Since such evaluation is based on the user's subjective view point, the evaluation becomes somewhat ambiguous. Further, accurate evaluation of thread tension/tension balance is a troublesome task for inexperienced users, and thus, may often lead to sewing operations executed under improper tension balance.
To address such problems, sewing machines have been conceived that makes automatic adjustments in tension balance. Such sewing machines are provided with a needle thread stitch detector and a bobbin thread stitch detector at the sewing machine body for optically detecting where, in the thickness of the workpiece cloth, the seam is formed, in other words, the depth in which the needle thread and the bobbin thread are interlaced to form a loop.
The above mentioned needle thread and bobbin thread detectors impinge a slit light on the seams of the workpiece cloth and detect the reflective light with a line sensor. Then, based on the detection of the line sensor, the height of the contour of the seam is detected to determine the depth of the seam or the point where the threads are interlaced from the detected height. However, the problem with such approach of detecting the height of the contour of the thread by way of a reflective detector is that precision in detecting the depth of interlace is readily affected by ambient light and the features of the workpiece such as color, design, and surface brilliance and thus could not sufficiently improve the quality of detection. Thus, evaluation of tension balance was not precise enough under the conventional configuration.